This invention relates to a rotary drum type dehydrator for dehydrating sludge through a rotary drum filter.
Known in the art of dehydrators is a rotary drum type dehydrator which dehydrates sludge by means of suction force provided by vacuum which is produced in the rotary drum filter. This type of dehydrator has, as illustrated in FIG. 6, a rotary drum filter a which is partly submerged in slurry contained in a slurry tank f and has a cylindrical filter made of a suitable filter material such as a wedge wire, a receiver tank b which is connected to the rotary filter drum a and a vacuum producing pump c connected to the receiver tank b. Liquid content of the sludge is sucked into the rotary drum filter a by means of suction force produced by the vacuum pump 3 and collected in the receiver tank b and solid content in the sludge is left in the form of cake on the outer surface of the rotary drum filter a and this cake is scraped off from the outer surface of the rotary drum filter a by a scraper.
A large vacuum producing pump is required as the pump c because vacuum must be produced in the rotary drum filter a through the receiver tank b. The dehydrator requires also a large tank as the receiver tank b because the receiver tank b must have a relatively large distance between the surface of filtered liquid and an inlet of a suction hose d for preventing the vacuum producing pump c from sucking liquid particles of the filtered liquid into the pump c. The receiver tank b must also have a relatively large distance between an outlet of a suction hose e connecting the rotary drum filter a and the receiver tank b and the inlet of the suction hose d for likewise preventing the vacuum producing pump c from sucking liquid particles of the filtered liquid into the pump c. Thus, the prior art rotary drum type dehydrator as a whole requires a relatively large and bulky apparatus which requires a high manufacturing cost.
In addition to such problem in design, space and cost, there is a serious technical disadvantage in this type of prior art dehydrator.
For producing suction force in the entire peripheral surface of the rotary drum filter, connecting portions between the rotary drum filter a, suction hose e, receiver tank b, suction hose d and vacuum producing pump c are completely sealed. After start of the dehydrating operation, air is sucked into the inside of the rotary drum filter a until a layer of slurry is formed around the entire outer cylindrical surface of the rotary drum filter a and, in this state, a high vacuum is not produced in the rotary drum filter a and the receiver tank b.
Upon completion of forming of the layer of slurry around the entire outer surface of the rotary drum filter a by rotation of the rotary drum filter a, sucking of air into the inside of the rotary drum filter a is suddenly interrupted and vacuum in the rotary drum filter a, instantaneously becomes high. This sudden increase in the degree of vacuum causes rapid compression of the slurry layer formed on the outer surface of the rotary drum filter a with the result that a layer of compact cake is instantaneously formed. This causes further interruption of air sucked into the rotary drum filter a through the layer of cake and this in turn further increases the degree of vacuum in the rotary drum filter a resulting in further compression of the cake layer. Thus, liquid content of slurry newly collected upon the compact cake layer cannot be sucked into the rotary drum filter a through the compact cake layer which forms a kind of cylindrical wall around the rotary drum filter a except in a case where there occurs a crack in the compact cake layer. Besides, such compact cake tends to block slits or pores of the filter material of the rotary drum which hampers the efficiency of the dehydration work.
This is particularly so in a case where slurry to be dehydrated is one of a high viscosity such as slurry containing bentonite. In the case of slurry containing bentonite, a thin film of compact cake is formed on the outer surface of the rotary drum filter whereby forming of a cake layer outside of the initially formed cake layer is made extremely difficult and blocking of filter is caused.
For this reason, a sufficient dehydrating effect cannot be attained by the prior art rotary drum type dehydrator and, in the industry, this type of dehydrator utilizing vacuum produced in the rotary drum filter is being replaced by other types of dehydrators such as those utilizing pressing of slurry and separating liquid content from solid content by centrifugal force.
It is, therefore, desirable for the rotary drum type dehydrator utilizing vacuum to provide vacuum in the rotary drum filter of such a degree that it does not increase suddenly but increases gradually after completion of forming of a layer of slurry around the outer surface of the rotary drum filter so that the slurry layer is not suddenly compressed to an excessively compact cake but is compressed to the degree that liquid content of slurry collected subsequently on the initially formed cake layer can be sucked into the rotary drum filter through the initially formed cake layer. Alternatively stated, vacuum in the rotary drum filter must be of such a degree that, after completion of forming of a slurry layer around the entire outer surface of the rotary drum filter, the initially formed cake allows dehydration of slurry collected subsequently on the initially formed cake.
The prior art rotary drum type dehydrator requires generation of high vacuum because of the above described design using the receiver tank between the rotary drum filter and the vacuum producing pump and it is extremely difficult for this type of dehydrator to provide moderate vacuum which, after completion of forming of a slurry layer around the outer surface of the rotary drum filter, allows dehydration of slurry collected subsequently on the initially formed cake.
It is, therefore, an object of the invention to provide a rotary drum type dehydrator capable of allowing, after completion of forming of an initial cake layer around the outer surface of a rotary drum filter, dehydration of slurry collected subsequently on the initially formed cake.
It is another object of the invention to provide a rotary drum type dehydrator which is of a compact and low-cost design while attaining dehydration of slurry effectively.